The present invention relates to a rotational angle detecting apparatus with a signal generator such as a crank angle detecting apparatus for an internal combustion engine in which the output signal of the signal generator, which is generated in synchrony with the rotation of a rotating shaft such as an engine crankshaft, is converted or waveform shaped into a square pulse signal representative of a predetermined rotational position of the rotating shaft. More specifically, it relates to a rotational position detecting apparatus in which the conversion efficiency of the output signal or the signal generator is improved so as to enhance the detection accuracy and reliability.
Generally, various operations such as ignition, fuel injection, etc., of an internal combustion engine for vehicles are controlled based on the rotational position or crank angle of the engine crankshaft. For example, ignition timing is controlled such that ignition takes place at a specific rotational position of the crankshaft. To this end, a rotational position detecting apparatus is employed for precisely detecting the rotational position or crank angle of the crankshaft.
A typical example of such a rotational position detecting apparatus is illustrated in FIGS. 6 through 11. As shown in FIG. 6, a rotating shaft 1, which is operatively connected with the crankshaft (not shown) of an internal combustion engine, is rotatably supported by a housing 9 through a bearing 9a. A rotating plate 2 is fixedly mounted on the rotating shaft 1 for integral rotation therewith. As shown in FIG. 7, the rotating plate 2 has a plurality of arcuate or sector-shaped slits 10 formed therein. The slits 10, the number of which corresponds to that of cylinders of the engine, are disposed on a circle around the axis of the rotating shaft 1 at locations circumferentially spaced from each other at substantially the same intervals. In the illustrated example, the number of slits 10 are four for a four-cylinder internal combustion engine.
A light emitting diode 3 and a light sensor 4 in the form of a photodiode are disposed on the opposite sides of the rotating plate 2 in alignment with each other on the circle on which the slits 10 in the rotating plate are disposed. The light emitting diode 3 and the light sensor 4 are fixedly supported by a holder 5 which is formed of a resin.
As clearly illustrated in FIGS. 8 and 9, the holder 5 includes a pair of support members 5a, 5a which are disposed in a confronting relation with each other and integrally connected with each other with a gap formed therebetween in which a portion of the rotating plate 2 is inserted. Each of the support members 5a, 5a has a generally cylindrical recess or socket 5b formed therein for receiving a corresponding light emitting diode 3 or a corresponding light sensor 4. Each of the recesses 5b is cylindrical or rectangular in vertical section. As shown in FIGS. 8 and 10, a rectangular-shaped window 11 is formed through the bottom portion of each cylindrical recess 5b so as to allow the light emitted from the light emitting diode 3 to reach the light sensor 4 after passing therethrough.
A circuit board 7 is mounted on an annular mounting plate 7a in the housing 9 and has an electronic circuit which is connected through wiring to the light emitting diode 3 and the light sensor 4 for driving the light emitting diode 3 and receiving an electric signal E from the light sensor 4. The electronic circuit of the circuit board 7 operates to shape the waveform of the electri signal E from the light sensor 4 into an appropriate pulse signal P.
The circuit board 7, the mounting plate 7a and a part of the holder 5 are received in a protective casing 8 which is received in and secured to the housing 9.
As illustrated in FIG. 11, the electronic circuit of the circuit board 7 includes a waveform shaper circuit having a comparator 20 which compares the electric signal E from the light sensor 4 with a reference voltage ER and generates a pulse signal P. The waveform shaper circuit includes a resistor 12 which is connected in series between the light emitting diode 3 and ground, a resistor 13 which is connected in series between the light sensor 4 and ground with a junction between the light sensor 4 and the resistor 13 being connected to the positive input terminal of the comparator 20, a pair of serially connected shunt resistors 14, 15 which are connected between the power supply and ground and have a junction therebetween connected to the negative input terminal of the comparator 20 for supplying the reference voltage ER thereto, and a resistor 16 which is connected to the output terminal 21 of the comparator 20.
The operation of the above-described rotational position detecting apparatus will now be described with particular reference to the waveform diagram of FIG. 12 as well as the characteristic graph of FIG. 13.
The light emitting diode 3 is powered from the electronic circuit of the circuit board 7 and is thus operated to generate light toward the light sensor 4. The light from the light emitting diode 3 intermittently passes through or is interrupted by the rotating plate 2, so that the light sensor 4 receives the light from the light emitting diode 3 at timings at which one of the slits 10 passes between or comes into alignment with the pair of light emitting diode 3 and light sensor 4. The light sensor 4 converts the light thus intermittently received from the light emitting diode 3 into an electric signal E which has a magnitude proportional to the quantity or magnitude of the received light. The light sensor 4 outputs the electric signal E thus generated to the electronic circuit of the circuit board 7.
At this time, however, the quantity of light passing through one of the slits 10, which becomes aligned with the light emitting diode 3 and the light sensor 4, changes substantially in a trapezoidal form in accordance with the rotational angle .theta. of the rotating plate 2 and the planar configurations of the slits 10 and the windows 11 in the support members 5a, 5a of the holder 5, so the electric signal generated by the light sensor 4 also changes substantially in the trapezoidal form, as depicted by the solid line in FIG. 12. Since the quantity of light passing through one of the slits 10 gradually changes when the radially extending edges of each arcuate slit 10 cross or superpose the edges of each window 11 in the circumferential or rotating direction of the rotating plate 2 (i.e., upon the opening and closing of each slit 10 with respect to the windows 11), as shown in FIG. 10, the rising and falling edges of each trapezoidal pulse of the light sensor output signal E change in curved lines, as shown by the two-dots and dashed line in FIG. 12, and hence the rate of change of the electric signal E becomes small particularly in the vicinity of the reference voltage ER.
On the other hand, the quantity of light received by the light sensor 4 changes according to the length or distance of a gap G between the light emitting diode 3 and the light sensor 4 (see FIG. 9) in a manner as illustrated in FIG. 13, i.e., substantially in an inverse proportion to the gap distance G. Thus, the quantity of light received by the light sensor 4 increases as the gap distance G decreases. In this connection, however, the holder 5 is formed by resin molding, so it is difficult to reduce the thickness of each window 11 (i.e., the thickness of the bottom portion of each socket 5b) from the points of view of manufacturing techniques and physical strength thereof. Therefore, there is a certain limit against reduction in the gap distance G.
The comparator 20 in the electric circuit 7 compares the electric signal E from the light sensor 4 with the prescribed reference voltage ER, and generates a pulse signal P which changes into the high level when the electric signal E exceeds the reference voltage ER, as shown in FIG. 12. Thus, the pulse signal P represents the rotational angle (rotational position) .theta. of the rotating plate 2.
With the conventional rotational position detecting apparatus as constructed above, however, the gap distance G between the light emitting diode 3 and the light sensor 4 is relatively large, and hence the magnitude of the electric signal E generated by the light sensor 4 is limited, resulting in a low efficiency in the photo-electric conversion. In addition, due to an inconsistency in configuration between the edge portions of the widows 11 and those of the slits 10, the rate of change of the electric signal E at the rising and falling edges in the vicinity of the reference voltage ER is relatively small, that is, the electric signal E does not rise or fall sharply in the vicinity of the reference voltage ER. As a result, if the quantity of light emitted by the light emitting diode 3 changes due, for example, to variations in the temperature of the ambient atmosphere, variations in the voltage supplied to the light emitting diode 3, etc., or if noise is superposed on the electric signal E of the light sensor 4, the pulse signal P generated by the waveform shaper circuit of FIG. 11 is easily affected, resulting in reduction in the detection accuracy. Particularly, in the case where noise is input to the positive input terminal of the comparator 20, the comparator 20 may malfunction and the instant or timing at which the pulse signal P rises or falls may greatly fluctuate, causing an error in the detection of the rotational angle or position of the crankshaft.
Further, in the conventional rotational position detecting apparatus as described above, since light is transmitted from the light emitting diode 3 to the light sensor 4 through the rectangular-shaped windows 11 and the arcuate or sector-shaped slits 10, the rising and falling portions of the electric signal E generated by the light sensor 4 have a limited rate of change in the vicinity of the reference voltage level ER. Accordingly, there is a problem in that a slight change in the quantity of light due to noise, variations in temperature and the like could easily result in detection errors.
Moreover, since the configuration of each recess or socket 5b is cylindrical or rectangular in vertical section, there is a certain minimum limit to the required thickness of the bottom of the recess of each support member 5a from the standpoint of mechanical strength, so it is difficult to decrease the gap distance G between the light emitting diode 3 and the light sensor 4, and hence a sufficient quantity of light for detection could hardly be provided at the light sensor. This leads to a low efficiency in the conversion of the light, which has passed through a slit 10, into an electric signal E, thus posing the problem that an SN ratio is degraded, preventing the detection of the crank angle or rotational position .theta. of the crankshaft with a high degree of accuracy.